The invention relates to a method for regulating the power supplied to a steam turbine by a pressurized water nuclear reactor driving an electrical generator supplying a network.
In pressurized water nuclear power stations used for the generation of electric current, steam is produced by heat exchange between the primary fluid constituted by water under pressure and the secondary fluid constituted by water which is converted into steam inside the steam generators.
The steam is used as a driving fluid for the turbine which actuates the electric current generator.
The electric power demanded by the network can be variable in the course of time, so that the power that the nuclear reactor must supply to the turbine may itself be essentially variable in the course of time. On the other hand, it is sometimes necessary to disconnect the current generator from the network for a longer or shorter time for a reason independent of the operation of the nuclear power station itself. In this case, the power demanded by the turbine is extremely low since it then suffices to supply the electrical power necessary for feeding the auxiliary circuits of the power station, the discharging network being disconnected. Generally, this residual power only represents about 5% of the rated power of the power station.
It is known that the operation of nuclear reactors to follow a given power program uses neutron-absorbant rods which are moved in the core of the reactor according to the power demand program.
However, in the case of considerable and rapid power variations to respond to the demand of the network, or in the case of electrical disconnection between the power station and the network, this operation being called load-rejection, it is necessary to have an additional regulating means for the power supplied by the steam turbine nuclear reactor.
The pressurized water nuclear power stations therefore include a steam by-pass system which permits an artificial load to be created capable of absorbing the excess power from the nuclear reactor. This artificial load is obtained by releasing steam tapped upstream of the turbine either to the atmosphere, or to the condenser.
Steam by-pass systems permit negative variations in the load of the turbine, of amplitude greater than 10% of the rated load, or a variation in the time of this load on the turbine, with a negative slope greater than 5% of the rated load per minute.
When the power program requires variations in load of smaller amplitude, compensation is effected only by movement of the control rods.
The by-pass systems also enable excess power to be absorbed on sudden reduction of power from the rated power to the power necessary for supplying the auxiliary circuits, namely, approximately 5% of the rated power, on load-rejection producing switching off of the turbo-generator unit without emergency shutdown of the nuclear reactor.
In the case of an emergency shutdown of the reactor, the by-pass system also serve for removing the heat and energy stored in the primary circuit and for bringing back the power station to zero load conditions, without it being necessary to open the safety valves of the steam generators.
The by-pass systems are constituted by valve units positioned in the secondary circuit of the reactor upstream of the steam inlet to the turbine. These valves are actuated electrically by means of an opening signal formed from measuring signals representing the values of certain parameters representing the load condition of the turbine or of the reactor.
The parameters used may be homogeneous with temperatures or with pressures and, as the case may be, one speaks of temperature-mode or pressure-mode regulation.
A conventional temperature-mode regulation method uses a pressure measurement on the first wheel of the turbine and forms from the signal representing this measurement a signal representing a parameter homogeneous with a temperature called reference temperature. This reference temperature is used for the regulation of the displacements of the control rods and also serves as a reference temperature for the by-pass control.
On the other hand, the average temperature in the primary circuit of the reactor is measured, and from the difference between the by-pass reference temperature and the average temperature of the primary circuit compensated by an advance-retard filter, an error signal is determined which is converted by a function generator into an opening signal for the by-pass.
When isolation of the power station is effected, the power of the reactor must be brought back from the rated power to a very low power just sufficient to maintain the supply of the auxiliary circuits.
This power is of the order of 5% of the rated power in the majority of cases.
The reactor is then controlled by means of the control rods to supply this power to the turbine, the by-pass valves being closed since the reference temperature of the by-pass is identical with the reference temperature for the operation of the reactor.
For such a low power demand from the reactor, the automatic power regulation cannot be placed in service and the operator must take control of the reactor manually, as soon as the power is less than a certain threshold.
On the other hand, when the connection with the network is restored, the power of the reactor must increase in the same way as the power supplied to this network during the increase in power.
This method of operation obviously has drawbacks.